Re: [Xen-devel] [PATCH v4 8/9] arm/mem_access: Add short-descriptor based gpt

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi Julien,

[...]

> 
> Looking at the code, I see very limited point of having the offsets
> array as you don't use a loop and also use each offset in a single place.
> 
>> +        ((paddr_t)(gva >> 20) & ((1ULL << (12 - n)) - 1)),
> 
Don't you think it is more readable to have the GVA offsets at one
place. Also, the functionality is in this way similar to the one shown
in guest_walk_ld. In my opinion, it is more intuitive to have both
functions work in a similar way. I suggest keeping the array, however
using GENMASK to generate it (as you mentioned in your comments below).

const vaddr_t offsets[2] = {
    (gva & GENMASK((31 - n), 20)) >> 20,
    (gva & GENMASK(19, 12)) >> 12,
};

> 
> Furthermore, it would be clearer if you first mask then shift. As it
> helps to understand the code.
> 
> If you use GENMASK (or GENMASK_ULL if you don't extend GENMASK), this
> will make everything more obvious:
> 

[...]


>> +
>> +    /*
>> +     * As we have considered up to 2 MSBs of the GVA for mapping the
>> first
>> +     * level translation table, we need to make sure that we limit
>> the table
>> +     * offset that is is indexed by GVA<31-n:20> to max 10 bits to avoid
>> +     * exceeding the page size limit.
>> +     */
>> +    mask = ((1ULL << 10) - 1);
>> +    pte = table[offsets[level] & mask];
> 
> This looks quite complex for just reading a pte. I think it would be
> worth to leverage the vgic_guest_access_memory for that (same in LPAE).
> It would also add safety if the offsets the table is mistakenly computed
> (from the current code, I can't convince myself the offset will always
> be right).

As far as I understand, we would still need to use the same offsets even
if we used vgic_access_guest_memory. Also, the only significant
difference between my implementation and vgic_access_guest_memory is
that gic_access_guest_memory checks whether we write over page
boundaries, which is quite hard to achieve as the offsets are limited in
number so that they don't cross page boundaries.

Yet, if you insist, I will try to incorporate vgic_access_guest_memory
into my implementation.

Thanks,
~Sergej



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